Electric photometer



March 1939- D. E. CHRISTOPHER ELECTRIC PHOTOMETER Filed May 21. 1937 FlG I FIG. 4

INVENTOR ATTORNEY Patented Mar. 21, 1939 UNITED STATES PATENT OFFICE 3Claims.

This invention relates to an improved electric photometer and has, for one .of its principal objects, the provision of an instrument for measuring low light intensities over small areas and which is particularly adaptable for use in the photographic industry.

One of the principal objects of this invention is to provide a photometer which operates on the principle of comparing a known light value with an unknown light value and can be used to advantage in the process of timing exposures for photographic prints.

Another important object of the invention is the provision of an electric photometer which can be of a relatively simple and economical construction but which will be sufliciently accurate for practically all purposes for which such devices are ordinarily used.

A further object is to provide, in connection with a photometer of this type, a scale wherein relative multiples of time or any other desired factor may be readily and accurately determined.

Other and further important objects of the invention will be apparent from the disclosures in the accompanying drawing and following specification.

The invention, in a preferred form, is shown in the drawing and hereinafter more fully described.

In the drawing:

Figure 1 is a top plan view of the improved electric photometer of this invention, illustrating a preferred embodiment thereof.

Figure 2 is a sectional view taken on the line A-A of Figure 1.

Figure 3 is a front elevation of the device shown in Figure 1.

Figure 4 is a horizontal section taken on the line BB of Figure 3, parts being broken away.

Figure 5 is an enlarged section on the line C-C of Figure 1.

Figure 6 is a similar section on the line D-D of Figure 1.

Figure 7 is a top plan view, showing a modified form of the invention.

Figure 8 is a vertical section on the line E-E of Figure 7.

Figure 9 illustrates another modification.

As shown in the drawing:

The reference numeral l0 indicates generally the casing of the improved electric photometer of this invention, the same comprising essentially a light-tight box provided on its top with a rotatable knob Hla. This knob surmounts a scale H which itself is rotatable with respect to the casing Ill and which can be composed of any suitable material.

As illustrated, the scale is graduated for an immediate time indication, namely, seconds and minutes, and in a logarithmic relationship, starting from an indicated exposure of one second and up to a time of from nine to ten minutes. The use of such a rotatable logarithmic scale renders an accurate multiplication of the time values relatively simple and easy, as a movement of the scale with regard to the arrow indicated at it permits multiplication by geometric addition of this logarithmic scale with a similar logarithmic scale( to the same base) of the factors employed.

As best shown in Figure 2, the casing Ill incloses a source of light l1, and the knob Illa is mounted on a post or shaft l9 rotatable in the casing and extending almost to the bottom thereof. At the lower end of this shaft i9 is fixed a filter I 8 which is made of any translucent material and comprises a series of areas graduated from light to dense as best indicated in Figure 4. The' areas need not be defined by lines but may gradually merge into each other starting with a space which is perfectly translucent and terminating in a space which is quite opaque, this being indicated by the variable spacing of the lines on the filter l8 as shown in Figure 4. This filter is arranged on the shaft I9 so that its original position will correspond with the original setting position of the scale I I.

Light from the source I! passes through the filter l8 onto a strip or pane of glass I2 as best shown in Figures 2 and 4, that portion of the glass not covered by the filter l8 being protected by a shielding ledge or the like 20.

The slide of glass 12 is backed by an opaque white surface or coating l3 (Figure 2) which refiects the light which is passed through the filter l8 into the glass, and this accordingly illuminates field 22 at the forward edge of the glass in accordance with the known phenomenon that some of the light rays entering a sheet or pane of glass will be reflected back and forth from the inner faces of the surfaces and will accordingly travel throughout the entire extent of the sheet making objects or markings anywhere on the surface more visible by indirect illumination. The forward edge of the glass slide l2 projects slightly beyond the casing l0 and is, in turn, inclosed in a housing 30 forming an extension of the bottom of the casing Ill.

The light of unknown intensity falls on the field 26 which reflects all light except that coming through the slide [2 and illuminating the field outlined by the opening 25. This light, of course, originates at the source I! and is passed through the filter it onto, into and throughout the slide [2. By regulating the intensity of the light source either directly or by means of a filter such as that shown at IS, the light intensity of the central field 25 may be varied; thereby the intensity of the illumination of the field 25 may be made to match that of the field 26, at which point the two fields appear as one. For any other unknown and the known light upon another in close proximity thereto and regulating the intensity of the latter until its reflection equals in intensity that of the unknown light, relative light values may accordingly be determined over a broad range, and by providing a suitable scale the relative light values can be compared with a standard of measurement whereby desired readings may be immediately and directly made.

These two varying fields 23 and 26 are simple uncoated or uncovered portions of the surfaces of the slide l2 and are provided so that the device may be equally adaptable to the measurement of direct light as in contact printing of photographs and also to the measurement of reflected light as in projection printing.

In measuring direct light, theopening 2| is placed over the area to be measured, and the intensity of field 22 is controlled by manipulating the knob iila until the reflected light'equals the intensity of the direct light whereupon the previously visible spot outlined by the opening 23 formed in the opaque white coating i3 disappears.

In measuring reflected light, the opening 24 is placed under the area to be measured, and the light on the field 22 is controlled until the intensities of fields 22 and 26 are equal. Such equalization of the light intensities, or, in other words, the illumination of the visible spots by control of the light intensity resulting from rotation of the knob Illa will immediately indicate to the operator the time in minutes or seconds required for the exposure of a negative or the like for proper photographic printing or development.

The density of the areas of the variable filter it are controlled so that the scale readings progress as the logarithms of the relative light values indicated. In other words, the relative light values are directly proportional to the time of exposure and may be substituted. A rotation and subsequent setting of the scale II to a position so that any known paper speed factor coincides with the arrow It will immediately indicate a reading for a light value which will be the time required for proper exposure of a particular paper. This direct multiplication is quite a convenience inv handling photographic paper and other factors.

In Figures 7 and 8 are illustrated a slightly different means for varying the intensity of the known light.

Here a casing 42 is provided having a slot 48 in its.top in which is slidably mounted a control knob or the like 44 which carries a source of light 45, the source of light being inside the casing.

At the bottom of the casing is provided a glass slide 43 similar to the slide I2, and the remainder of the apparatus is substantially the same as that shown in Figures 5 and 6.

The knob 44 is provided with a pointer which moves over a scale 48 which may be graduated into any desired time periods.

In Figure 9, the casing 50 has mounted in its top a rotatable resistance knob 5! which controls a rheostat or variable resistance 41 whereby the intensity of the light source 52 is regulated, this, in turn, operating directly in conjunction with a glass slide or the like 53. A circular logarithmic scale similar to the scale ll may be employed in this connection.

It will be seen that herein is provided a simple yet eflective electric photometer which will be especially convenient and adaptable for photo graphic and similar work and which is so simple in operation that unusually accurate results can be immediately obtained by relatively unskilled persons. 7

Another distinct advantage of this device lies in the ability of the operator to accurately measureeither direct or reflected light at substantially exactly a desired point or over very small areas, and directly on and closely adjacent to a designated surface. Errors are accordingly reduced to a minimum.

I am aware that many changes may be made and numerous details of construction varied throughout a wide range without departing from the principles of this invention, and I, therefore, do not purpose limiting the patent granted hereon otherwise than as necessitated by the prior art.

I claim as my invention:

1. A light meter, comprising a light-tight casing, a source of light therein, a pane of glass in the bottom of the casing. one end of said glass pane extending beyond the casing, an opaque coating on the projecting portion of the glass, said coating having an opening therein through which light in the glass is visible, means for varying the amount of light falling on the glass pane, said means comprising a translucent filter of varying density, means for rotating said filter between the source of light and the glass pane, said means comprising a shaft extending through the box cover, an operating knob on the upper end of the shaft, and a logarithmic scale graduated in time periods on the box cover beneath the knob, said scale being rotatable about the shaft and with respect to an indicated point on the box cover.

2. A photometer, comprising a casing, a source of light in the casing, a pane of glass in the bottom of the casing, means for varying the amount of light falling on the glass, said glass pane projecting beyond the casing, a double cover for the projecting end, openings in the cover through which the glass is exposed, opaque coatings on those portions of the glass beneath the openings, said coatings being cut away to provide small holes in approximately the center 01 the casing openings, one of said openings exposing the top of the glass only while the other opening exposes both faces of the glass, the opaque coating and central hole being on the bottom surface of the glass in said last-named instance.

3. A photometer, comprising a light-tight casing, a source of light therein, a comparison field in the casing, said comparison field including a translucent spot surrounded by a reflecting area,

means for varying the amount of light falling on 

